UC Davis

Microalgal-bacterial aggregates (MBAs) have recently attracted significant attention as a potential replacement for conventional, suspended-growth wastewater treatment. Proponents of MBAs often claim several key benefits: 1) elimination or reduction of external aeration requirements, which would greatly reduce energy consumption, 2) improved resource recovery through the production of value-added products from microalgal biomass, and 3) removal of nitrogen, phosphorus, and bioavailable chemical oxygen demand (bCOD) that is competitive with conventional technologies (e.g., activated sludge). This article briefly reviews the development of MBAs and evaluates their feasibility for full-scale implementation. The results suggest that MBAs and similar suspended-growth processes are functionally identical to well-mixed wastewater treatment ponds, which have been used and studied for decades and require a substantial amount of surface area to treat typical municipal wastewater. We estimate that photosynthesis and atmospheric diffusion would provide at most only 2.7% of the oxygen required for bCOD removal in a nitrifying activated sludge process. Thus, MBAs and similar treatment processes are not viable alternatives to conventional wastewater treatment processes when space is limited. However, attached-growth bioreactors designed to maximize atmospheric diffusion of oxygen relative to their footprint are promising for compact, low-energy wastewater treatment with microalgal-bacterial consortia.